1. Field of the Invention
The invention concerns cathode ray tubes and, more precisely, the making of the cathode of these tubes.
2. Description of the Prior Art
The cathode is that element of the tube which enables the emission of an electron beam towards the display screen of the tube.
To produce this emission of electrons, the cathode has, in its front part pointed towards the screen, a substance chosen for its capacity to emit a high density of electrons when it is raised to a sufficient temperature and when it is subjected to a strong electrical field.
The active substance is most usually heated by an electrically resistive filament (a wound tungsten filament) placed inside a sleeve, one end of which carries the active electron emitting substance. Herein, we shall be concerned only with filament heated cathodes although there are also so-called directly heated cathodes having no filament.
Depending on the applications envisaged (in particular, depending on whether a greater or smaller electrical emission density is desired) two types of cathodes are commonly used.
The first type of cathode is called the "impregnated cathode". It is made in the following way: the active electron emitting element, placed in the front of the cathode, is a chip made of porous, sintered tungsten, the pores of which are filled with barium-calcium aluminate. It is this substance that forms the emissive material. Emission takes place as a result of the chemical reaction between tungsten and aluminate, and this reaction produces free barium. Since the extraction potential of barium electron towards the vacuum is low, the emission of electrons becomes possible. However, this chemical reaction occurs only at a very high temperature of about 1100.degree. C., so that the filament which will be used to produce this temperature should itself be carried to an even greater temperature. These impregnated cathodes are very valuable because they can be used to give very high electron emitting density without any acceleration of ageing owing to this high density, but they are very costly because of the technological constraint imposed by the very high temperature of the filament and because of difficulties in manufacturing (machining, impregnating, etc.) the active chips of impregnated tungsten.
The second type of commonly used cathode is called the "oxide cathode". The active emitting element herein is a capsule of active nickel (nickel with the addition of a small percentage of tungsten and traces of magnesium) coated, on its front side pointed to the screen, with a layer of barium, calcium and strontium oxides. The heating filament is contained in a nickel tube enclosed by the capsule of active nickel. The temperature of the nickel capsule should reach about 800.degree. C. so that the reactions needed for electronic emission occur. These reactions are, firstly, an oxidation-reduction of the barium, calcium and strontium oxides by the active nickel and, secondly, an electrolysis of the oxides layer to release barium. The advantage of these cathodes is their lower working temperature, whence their lower manufacturing cost, but their main drawback as compared with impregnated cathodes, is their far lower emitting density if it is desired that they should have a sufficient lifetime.
The present invention concerns solely oxide cathodes for applications in which the electron emission density does not need to be very high and for which, consequently, there would be no justification for the high cost of an impregnated cathode. The standard constitution of a prior art oxide cathode is shown in the sectional view in FIG. 1.
It has a metallic tube 10 or cathode tube containing the heating filament 12 (shown uncut). The tube is open at its rear end (on the side opposite to the electron transmission) to let through the ends 14 and 16 of the filament which should be connected to a voltage source.
The front end of the cathode tube 10 is enclosed by a capsule 18 of active nickel (nickel with the addition, for example, of 4% of tungsten and 0.05% of magnesium). This capsule fits on to the top of the tube, and its internal diameter is equal to the external diameter of the tube in such a way that the tube is hermetically sealed at its front end. The capsule is electrically spot-soldered all around its periphery.
A layer 10 of barium/calcium/strontium oxides is deposited on the external front surface of the capsule 18 of active nickel, namely on that portion of the tube-closing capsule which is outside the tube.
The cathode tube 10 is mounted on a disk-shaped insulating support 22, used to center the cathode tube in the cathode ray tube for which the cathode is the emitting element. The fixing of the tube 10 is done through a thermal shield 24, made of a nickel-chromium alloy for example, which concentrically surrounds the tube 10 without touching it in its hottest part (namely in the central part and front end of the tube 10) and which is soldered to the tube in its rear part towards the ends of the filament.
The thermal shield 24 is crimped onto the insulating support 22. Its function is to avoid heat losses from the cathode tube 10. It is not in contact with the cathode tube, except at one end, and besides the contact occurs only at some points, the cylindrical thermal shield being pinched in a triangular form to its rear end, so that it can be soldered at three spots to the rear part of the cathode tube. There is therefore little transmission of heat by conduction between the cathode tube 10 and the supporting element. Furthermore, the thermal shield is reflective to prevent heat losses by radiation. Finally, it will be noted that the wall of the cathode tube 10 is very thin, here again to prevent heat losses by thermal conduction.
The heating filament 12 is a tungsten wire or tungsten alloy wire in a double helix winding as can be seen in FIG. 1. Sometimes, this coil is made not on a single, linear wire of tungsten but on a wire which is already in a helix winding with small-diameter turns, in order to increase the length of the wire.
The tungsten wire is not a bare wire, but a wire coated with a fine alumina layer (not seen in FIG. 1) so that the turns are electrically insulated from one another and from the cathode.
We have thus described the structure of a prior art oxide cathode.
The essential problem faced is the strength of the filament under severe environmental conditions, and especially in the presence of vibrations. The alumina sheath which coats the tungsten wire should be very pure and have no cracks to prevent breakdown and leakage currents, especially when the ends of the filament are carried to a potential which is very different from that of the cathode (in this respect, technical specifications often require the cathode to operate faultlessly, even when the cathode-filament potential difference reaches 200 volts).
When there are vibrations, the alumina sheath cracks and the risks of breakdown and leakages inside the cathode tubes are heightened. Furthermore, the wearing down of the alumina sheath produces particles which are propagated outside the cathode tube and penetrate the chamber of the cathode ray tube itself. The very high voltages (15 to 40 kilovolts) present in this chamber cause the breakdown of the cathode ray tube when foreign particles move about in the highly intense electrical fields that prevail between the anode and the cathode. Even if the cathode ray tube does not break down, the particles get deposited on the display screen, which is carried to very high voltage, and cause black spots thereon, spoiling the quality of the image.
To boost the mechanical strength of the filament, it has already been proposed to introduce, in the axis of the wound filament, a reinforcing strip soldered to the cathode tube. This strip is coated with alumina so that it remains electrically insulted from the filament. Then it is inserted within the tungsten wire coil. This insertion is a delicate task because it should not damage the fine alumina sheath of the reinforcing strip. Then the filament mounted on the strip is covered with alumina. Then the strip is electrically soldered to the cathode tube at its rear end. The strip is preferably U-shaped (like a hairpin). During operation, through it rigidity which is greater than that of the wound filament, it restricts the movements of the latter, and hence limits the wearing down of the sheath of the tungsten wire.
This type of cathode with a reinforced filament has the following drawbacks:
high cost (5 to 10 times the cost of an ordinary filament) because of additional assembly operations; PA1 difficulty of assembly in the cathode because the reinforcing strip of the cathode tube must be soldered without damaging the alumina sheath; PA1 average and undependable sturdiness; PA1 the positioning of the filament with respect to the closed end of the cathode tube is difficult and non-reproducible because the filament cannot be pushed completely up to the back of the tube (as could be done with a non-reinforced filament). For, subsequent expansions, during operation, of the reinforcing strip have to be taken into account. PA1 the preparation of a cathode tube made of refractory metal; PA1 the positioning of a plug, made of a refractory metal which is a good heat conductor, at the end which will become the front end of the tube, and the soldering of this plug to the inner wall of the tube, in letting a part of the plug extend beyond the front end of the tube; PA1 the coating of a heating filament with a refractory insulating material; PA1 then the positioning of the filament inside the plugged tube; PA1 the introduction in the tube of a suspension of refractory insulating powder, preferably the same as the one coating the filament; PA1 the evaporation of the suspension solvent; PA1 the sintering, at high temperature, of the insulating refractory powder to provide mechanical cohesion between the filament and the internal wall of the tube through the sintered powder; PA1 the positioning of an active nickel capsule on the refractory metal plug; PA1 and the soldering of this capsule to the wall of the tube at the front end of said tube facing the plug.
On this latter point (difficulty of positioning) it should be clearly understood not only that the temperature of the cathode is lower if the filament is not completely pushed in but, above all, that if the filament is not always placed strictly in the same position in a mass-produced batch of cathodes, the result will be variation in the cathode temperature reached during operation, hence variation in transmission characteristics. These variations are not acceptable.